Until now, cast steels have been produced by casting molten steel into slabs, blooms, billets and cast strips, etc. through ingot casting methods using fixed molds and through continuous casting methods using oscillation molds, belt casters and strip casters, etc. and by cutting them into prescribed sizes.
Said cast steels are heated in reheating furnaces, etc., and then processed to produce steel sheets and sections, etc. through rough rolling and finish rolling, etc.
Likewise, cast steels for seamless steel pipes are produced by casting molten steel into blooms and billets using ingot casting methods and continuous casting methods. Said cast steels are heated in reheating furnaces, etc., are then subjected to rough rolling, and are sent to pipe manufacturing processes as steel materials for pipe manufacturing. Further, the steel materials are formed into rectangular or round shapes after being heated again, and then are pierced with plugs to produce seamless pipes.
Solidification structures of cast steels before processing, as well as the conditions of processing such as rolling, etc., have a great influence on the properties and quality of the steel materials.
In general, the structure of a cast steel is, as shown in FIG. 7, composed of relatively fine chilled crystals in the surface layer cooled and solidified rapidly by a mold, large columnar crystals formed at the inside of the surface layer, and equiaxed crystals formed at the center portion. In some cases, the columnar crystals may reach the center portion.
When coarse columnar crystals exist in the surface layer of a cast steel as mentioned above, tramp elements of Cu, etc. and their chemical compounds segregate at the grain boundaries of the large columnar crystals, resulting in the brittleness of the segregated portions and the generation of surface flaws in the surface layer of the cast steel, such as cracks and dents caused by uneven cooling, etc. As a result, the yield deteriorates due to the increase of reconditioning work such as grinding and scrapping of the cast steel.
When processing the above-mentioned cast steel by rolling etc., since anisotropy of deformation caused by uneven crystal grain size becomes large, deformation behavior in the transverse direction becomes different from that in the longitudinal direction and the defects such as scabs and cracks, etc., are apt to arise. Further, forming properties such as the r-value (drawing index) deteriorate, and/or surface flaws such as wrinkles (in particular, ridging and roping in stainless steel sheets) appear.
In particular, in a stainless steel material in which the appearance is important, surface flaws such as edge seam defects and roping arise, leading to poor appearance and an increase in the edge trimming amount.
Further, when a seamless steel pipe is produced from the above-mentioned cast steel, surface flaws such as scabs and cracks or internal defects such as internal cracks, voids and center segregation caused by the cast steel remain in the steel pipe. Moreover, during pipe manufacturing, the above-mentioned defects are promoted by forming and piercing and defects such as cracks and scabs are generated on the inner surface of the steel pipe. This leads to the lowering of the yield due to the increase of reconditioning such as grinding or the frequent occurrence of scrapping.
This tendency appears markedly in ferritic stainless seamless pipes containing chromium.
When coarse columnar crystals and large equiaxed crystals exist at the interior of a cast steel, internal defects, such as internal cracks resulted from strain imposed by bulging and straightening, etc., center porosity resulted from the solidification contraction of molten steel and center segregation caused by the flow of unsolidified molten steel at the last stage of solidification, are generated in the cast steel.
Thus the surface flaws generated on a cast steel cause the deterioration of yield caused by an increase in reconditioning work such as grinding and the frequent occurrence of scrapping. If this cast steel is used as it is for processing such as rough rolling and finish rolling, etc., in addition to the surface flaws generated on the cast steel, internal defects such as internal cracks, center porosity and center segregation, etc., remain in the steel material, resulting in the rejection by UST (Ultrasonic Test), the degradation of strength or the deterioration of appearance, and consequent increase of reconditioning work and frequent occurrence of scrapping of the steel material.
Surface flaws and internal defects in a cast steel can be suppressed by improving the solidification structure of the cast steel.
Further, the generation of surface flaws such as surface cracks and dents caused by uneven cooling and uneven solidification contraction arising in a cast steel can be suppressed by making the solidification structure of the cast steel uniform and fine.
Moreover, the generation of internal defects such as internal cracks, center porosity and center segregation, etc., caused by the solidification contraction and the flow of unsolidified molten steel, etc. at the interior of the cast steel can be suppressed by raising the equiaxed crystal ratio at the interior of the cast steel.
Therefore, to suppress the occurrence of surface flaws and internal defects of a cast steel and a steel material produced therefrom and improve the workability and quality such as toughness, etc., of the cast steel, it is important to suppress the coarsening of columnar crystals at the surface layer of the cast steel, to raise the equiaxed crystal ratio at the interior of the cast steel, and to make a uniform and fine solidification structure as a whole.
To cope with these problems, various measures for preventing the occurrence of surface flaws and internal defects in a cast steel and a steel material produced therefrom, such as to devise the form of inclusions in molten steel and to make a solidification structure into fine equiaxed crystal structure by controlling solidification process, have been attempted.
By the way, as measures to raise an equiaxed crystal ratio in the solidification structure of a cast steel, known are (1) a method for casting at a low temperature by lowering the temperature of molten steel, (2) a method for electromagnetically stirring molten steel in solidification process, and (3) a method for generating oxides and inclusions in molten steel by adding themselves or other components in molten steel to act as solidification nuclei at the time of the solidification of molten steel, or a method combining the above methods (1) to (3).
As an embodiment related to low temperature casting by the above method (1), for example, disclosed is a method in Japanese Examined Patent Publication No. 7-84617 for preventing ridging from occurring on a ferritic stainless steel sheet by extracting a cast steel while cooling it in a mold and maintaining the superheat temperature (a temperature obtained by subtracting liquidus temperature of molten steel from actual temperature of molten steel) at not more than 40° C. while continuously casting molten steel, and by maintaining the equiaxed crystal ratio of the cast steel to not less than 70%.
However, according to the method disclosed in Japanese Examined Patent Publication No. 7-84617, since the superheat temperature is lowered, there occur the problems of generating nozzle clogging caused by the solidification of molten steel during casting, making casting difficult due to the adhesion of skull, preventing the floating of inclusions caused by the increase of viscosity, and generating defects caused by inclusions remaining in molten steel. Therefore, by this method, it is difficult to lower the superheat temperature to the extent that a cast steel with sufficient equiaxed crystal ratio can be obtained.
Thus, it has not so far been clarified as to how large grain size of equiaxed crystals from the surface layer to the interior of a cast steel is desirable and how uniform the solidification structure should be.
In Japanese Unexamined Patent Publication No. 57-62804, a method is disclosed for reducing a cast steel and bonding the central area with pressure under the condition that unsolidified portions remain in the interior, in order to eliminate internal defects such as center porosity, etc. in the cast steel.
However, according to the method disclosed in Japanese Unexamined Patent Publication No. 57-62804, since the center area of a cast steel is bonded with pressure by reduction, when the unsolidified portion is large, the brittle solidified layer is subjected to a large reduction force, and this causes internal cracks and center segregation, etc. On the other hand, when the reduction is insufficient, there are problems that internal defects such as center porosity, etc. remain, and this causes the generation of defects on inner surface, such as cracks and scabs, when the cast steel is pierced in the pipe manufacturing process, which causes the deterioration of quality of the steel pipe.
As mentioned above, by those conventional methods, it is difficult to produce a chromium-containing cast steel having a fine solidification structure and controlled surface flaws and internal defects and further to produce a pipe without breaking down (applying large reduction to) the continuously cast steel. Moreover, it has not so far been clarified as to what kind of casting and treatment of a cast steel should be carried out for producing stably and industrially a pipe of chromium-containing steel (ferritic stainless steel) without defects.
Further, as a method for applying electromagnetic stirring to molten steel according to the above method (2), for example, as disclosed in Japanese Unexamined Patent Publication Nos. 49-52725 and 2-151354, there is a method for improving the solidification structure of a cast steel by applying electromagnetic stirring to molten steel in a mold or downstream of the mold during a solidification process, promoting the floating of inclusions and controlling the growth of columnar crystals.
However, according to the method disclosed in Japanese Unexamined Patent Publication Nos. 49-52725 and 2-151354, when a stirring flow is imposed on molten steel at the vicinity of a mold by electromagnetic stirring, though the solidification structure of the surface layer portion of a cast steel can become fine, that of the interior of the cast steel cannot become sufficiently fine. On the other hand, when a stirring flow is imposed on molten steel downstream of a mold, though the solidification structure of the interior of a cast steel can become fine, large columnar crystals are formed at the surface layer portion of the cast steel, and thus it is impossible to make the solidification structures of the interior and surface layer portions of the cast steel fine at the same time.
Moreover, by only imposing a stirring flow on molten steel during solidification process with electromagnetic stirring, it is difficult to obtain a cast steel having a fine solidification structure with a prescribed grain size, and thus the effect of electromagnetic stirring on the fining of a solidification structure is limited.
Further, as a method for applying electromagnetic stirring to molten steel, as disclosed in Japanese Unexamined Patent Publication No. 50-16616, there is a method for preventing ridging by applying electromagnetic stirring to molten steel during a solidification process, cutting the tips of growing columnar crystals, making use of the cut tips of the columnar crystals as solidification nuclei, and controlling equiaxed crystal ratio in the solidification structure of the cast steel to not less than 60%.
However, according to the method disclosed in Japanese Unexamined Patent Publication No. 50-16616, since electromagnetic stirring is applied to a cast steel leaving a mold, columnar crystals exist in the surface layer of the cast steel. Thus, on the cast steel, surface flaws such as cracks and dents caused by the columnar crystals occur, and on the steel material processed by rolling, etc., in addition to scabs and cracks, surface flaws such as ridging occur.
Yet further, there are methods, as disclosed in Japanese Unexamined Patent Publication No. 52-47522, for producing a cast steel with a fine solidification structure by installing an electromagnetic stirrer at a point 1.5 to 3.0 m distant from the meniscus in a continuous casting mold and stirring molten steel at a thrust of 60 mmHg, and, as disclosed in Japanese Unexamined Patent Publication No. 52-60231, for producing a steel material not having internal defects such as center segregation and center porosity, etc. by casting molten steel at the superheat temperature of 10 to 50° C., also applying electromagnetic stirring to unsolidified layer of a cast steel under casting, and making the solidification structure into fine structure composed of equiaxed crystals.
However, according to the method disclosed in Japanese Unexamined Patent Publication No. 52-47522, since growing columnar crystals (a dendrite structure) are suppressed by applying electromagnetic stirring to molten steel during solidifying in a mold, though the solidification structure near the portion where electromagnetic stirring is imposed can become fine to some extent, to make the whole solidification structure of the cast steel fine, there is still a problem that a multistage electromagnetic stirrer is necessary and thus the equipment cost increases. Moreover, the installation of a multistage electromagnetic stirrer is extremely difficult from the viewpoint of space for installation, and thus the method disclosed in Japanese Unexamined Patent Publication No. 52-47522 has a limitation in producing a cast steel a whole solidification structure of which is fine.
Further, according to the method disclosed in Japanese Unexamined Patent Publication No. 52-60231, since low temperature casting is applied, there are problems that nozzles clog due to the deposition of inclusions on the inner surface of an immersion nozzle, a skin is formed on the surface of molten steel due to the temperature drop of molten steel in a mold, and thus, in some cases, the operation becomes unstable and the casting operation is interrupted.
As mentioned above, in case of low temperature casting, because the temperature for casting molten steel is lowered, problems occur such as the interruption of casting caused by the clogging of an immersion nozzle used for pouring molten steel in a mold and the decline of casting speed caused by the decrease of the feed amount of molten steel, and thus it is difficult to lower the casting temperature to the extent capable of stably making the solidification structure of a cast steel fine.
Further, in case of using an electromagnetic stirrer, even though electromagnetic stirring is applied locally during the solidification of molten steel, there are drawbacks in that columnar crystals and coarse equiaxed crystals are generated and this causes surface flaws and internal defects, and thus yield deteriorates due to the increase of reconditioning and the frequent occurrence of scrapping and the quality of the steel material also deteriorates due to internal defects such as internal cracks and center porosity, etc.
On the other hand, it may be considered to make a solidification structure fine over the whole cross section of a cast steel by installing a plurality of electromagnetic stirrers at the downstream side of a mold including a meniscus. However, since the degree of fining varies depending on the portion where stirring is applied, it is impossible to stably obtain a fine solidification structure over the whole cast steel. If it is required to obtain a stable and fine solidification structure, the number of electromagnetic stirrers to be installed increases. Since the number of electromagnetic stirrers to be installed is restricted by equipment cost and the configuration of a continuous caster, the installation itself of the required number of stirrers is difficult. In any event, even though a plurality of electromagnetic stirrers are installed, sufficient fining of a solidification structure cannot be obtained.
Moreover, as an embodiment of a method for generating oxides and inclusions in molten steel, which act as solidification nuclei, by adding the oxides or inclusions themselves or other components into molten steel according to the above method (3), for example, disclosed is a method, in Japanese Unexamined Patent Publication No. 53-90129, for making whole solidification structure of a cast steel into equiaxed crystals by adding into molten steel a wire wherein iron powder and oxides of Co, B, W and Mo, etc., are wrapped and applying a stirring flow to the place where the wire melts. However, by this method, the dissolution of the additives in the wire is unstable and sometimes undissolved remainders appear. When undissolved remainders appear, they cause product defects. Even if all the additives in the wire are dissolved, it is extremely difficult to uniformly disperse the additives throughout the entire cast steel from the surface layer to the interior. As a result, the size of the solidification structure becomes uneven which is not desirable. Besides, since the effect of equiaxed crystallization is influenced by the position of an electromagnetic stirrer and the stirring thrust, this method has a drawback of undergoing constraint by conditions related to equipment. A method for adding fine particles of TiN, etc. during casting is disclosed in Japanese Unexamined Patent Publication No. 63-140061. However, this method has the same drawbacks as that of Japanese Unexamined Patent Publication No. 53-90129.
With regard to the effect of generating inclusions which act as solidification nuclei by adding required components in molten steel, for example, a method is generally known to form TiN in molten steel of ferritic stainless steel and to produce equiaxed crystals in the solidification structure (Tetsu to Hagane Vol.4-S79, 1974, for example). However, to obtain a sufficient effect of equiaxed crystallization by the formation of TiN as mentioned above, as described in above “Tetsu to Hagane,” it is necessary to increase Ti concentration in molten steel up to not less than 0.15 mass %.
Therefore, to obtain sufficient equiaxed crystallization by the formation of TiN as mentioned above, an increased addition amount of expensive Ti alloy is required, which leads to a higher manufacturing cost. Furthermore, there arise the problems of nozzle throttling caused by coarse TiN during casting and formation of scabs on the product sheet. Besides, since the chemical composition of the steel is restricted in relation to the addition amount of TiN, applicable steel grades are limited.
A means is desired for effectively obtaining a cast steel with a fine equiaxed crystal structure by adding some components in as small amounts as possible, and for that reason, a method to add Mg to molten steel is proposed.
However, since the boiling point of Mg is about 1,107° C., lower than the temperature of molten steel and the solubility of Mg in molten steel is almost zero, even if metallic Mg is added to molten steel, most of it is vaporized and escapes away. Therefore, if Mg is added by a usual method, the Mg yield generally becomes very low, and thus it is necessary to devise a means for Mg addition.
The present inventors, during the course of research on Mg addition, have found that the composition of oxides formed after Mg addition is affected by not only the composition of molten steel but also the composition of slag. That is, it has been found that, by only adding Mg to molten steel, it is difficult to form inclusions which have composition acting effectively as solidification nuclei in molten steel.
For example, in Japanese Unexamined Patent Publication No. 7-48616, disclosed is a method for improving Mg yield in molten steel by providing the slag covering the molten steel surface in a container such as a ladle with CaO—SiO2—Al2O3 slag containing MgO adjusted to 3 to 15 wt % and FeO, Fe2O3 and MnO adjusted to not more than 5 wt %, and adding Mg alloy passing through the slag, and also, for improving the quality of a steel material by forming fine oxides of MgO and MgO—Al2O3.
According to the method disclosed in Japanese Unexamined Patent Publication No. 7-48616, since the slag of CaO—SiO2—Al2O3 covers the surface of the molten steel, there is an advantage that the improvement of yield can be expected by suppressing the evaporation of Mg. However, by the method disclosed in Japanese Unexamined Patent Publication No. 7-48616, only the total amount of FeO, Fe2O3 and MnO in slag covering molten metal is specified to be not more than 5 wt % and the amount of SiO2 is not specified. Then, if SiO2 is abundantly contained in slag, when metallic Mg or Mg alloy is added, Mg reacts with SiO2 contained in slag and the Mg yield in molten steel drops. When the Mg yield is low, Al2O3, etc., in molten steel can not be reformed into oxides containing MgO, coarse oxides of Al2O3 remain in molten steel and this causes the generation of defects in a cast steel and a steel material after all.
Since the function of Al2O3 system oxides as solidification nuclei is limited, the solidification structure of a cast steel coarsens and defects, such as cracks, center segregation and center porosity, etc., arise on the surface or in the interior of the cast steel, and thus the yield of the cast steel deteriorates.
Further, there are problems that, in the steel material produced from the above cast steel too, surface flaws and internal defects caused by a coarse solidification structure arise, and thus yield and quality deteriorate.
Moreover, since no restrictions are specified for CaO concentration in slag or Ca concentration in molten steel, in some cases, instead of the generation of high-melting-point MgO, etc., low-melting-point complex compounds (CaO—Al2O3—MgO oxides) which do not act as solidification nuclei are generated.
In Japanese Unexamined Patent Publication Nos. 10-102131 and 10-296409, proposed are methods for improving the solidification structure of a cast steel by controlling the amount of Mg contained in molten steel at 0.001 to 0.015 wt %, forming fine oxides with good dispersibility, and distributing the oxides over the entire cast steel.
However, by the methods disclosed in Japanese Unexamined Patent Publication Nos. 10-102131 and 10-296409, since oxides are uniformly distributed from the surface layer portion to the interior of a cast steel at a high density of not less than 50/mm2, in some cases, defects such as cracks and scabs caused by oxides arise on the cast steel, the cast steel being processed or the steel material processed from the cast steel. In this case, reconditioning such as surface grinding, etc. is required or the steel material is scrapped, and thus the yield of products drops.
Further, when oxides are exposed on the surface of a steel material or exist in the vicinity of a surface layer, there are problems that, when the oxides touch acid or salt water, etc., oxides (oxides containing MgO) dissolve out and the corrosion resistance of the steel material deteriorates.
Then, as a result of carrying out various experiments to clarify the optimum conditions for equiaxed crystallization obtained by adding Mg to molten steel, the present inventors have newly found that, even though a molten steel component and/or a slag composition are not changed, the order of the addition of Mg and deoxidation elements such as Al has a great influence on the effect on equiaxed crystallization.
That is, it was found that, when Al is added after Mg is added to molten steel, since Al2O3 covers the surface of MgO generated after Mg addition, the generated MgO does not act effectively as a solidification nucleus.
As a result, the effect of MgO on making a solidification structure fine cannot be obtained, the solidification structure coarsens, and surface flaws such as cracks, etc. and internal defects such as center segregation and center porosity, etc. arise. As a result, reconditioning work of a cast steel and a steel material increases, a cast steel and a steel material are scrapped, and the yield and quality of products deteriorate.
As mentioned above, by conventional methods of adding oxides and inclusions themselves to molten steel as solidification nuclei, and generating solidification nuclei in molten steel by adding a required component, it is difficult to obtain a cast steel of a uniform solidification structure without defects. Therefore, there is a problem that it is impossible to obtain a cast steel with excellent workability during rolling, etc., and further a steel material with good quality and few defects.
It has so far not been clarified as to what kind of solidification structure should be obtained for stably and industrially producing a cast steel with good workability but without defects.
As explained above, the reality is that, with the conventional methods for obtaining equiaxed crystallization of a cast steel by casting at a low temperature, adopting electromagnetic stirring or adding oxides which form solidification nuclei, it is impossible to stably and industrially produce a steel material with excellent quality and few defects by suppressing the generation of surface flaws and internal defects such as cracks, dents, center segregation and center porosity, etc. which arise in a cast steel, and further obtaining a defect-less cast steel having a solidification structure with a uniform grain diameter, and thus improving the workability of the cast steel.